Since a wireless communication system has a limited available bandwidth, techniques using multi beam in the same channel have been studied in order to achieve a higher transmission rate. However, it is difficult to apply the technologies using the multi beam in the same channel to an actual circumstance due to a complex structure and a difficulty in a present circumstance.
Recently, a rotation mode (orbital angular momentum) of a multi beam antenna is actively studied. Using a fact that orthogonality is present between modes in the case of a rotation mode, a door to theoretically configure an infinite transmission channel is open. However, there is a limit in a technique of receiving a signal by applying the rotation mode to an actual wireless transmission channel.
For example, in the case of the rotation mode, a hole which does not have energy at a center is generated due to a radiation characteristic of an antenna and a radiation pattern expands in accordance with a distance. Therefore, there are lots of problems in a size of a reception antenna which receives a signal through the antenna.